This invention relates to polymeric packaging film, and more particularly to compositions and films having properties useful for stretch wrapping of palletized loads such as boxes, bags, and the like.
Conventionally, the polymeric films utilized for stretch wrapping of palletized loads are constituted of polyvinyl chloride, polyethylene, and ethylene/vinyl acetate copolymers. Although polyvinyl chloride film is a popular pallet stretch wrap material, it possesses a low ultimate tensile strength in elongation, poor puncture resistance, and high specific gravity which yields a low area of film per unit weight. Polyvinyl chloride film is especially poor in stress relaxation, losing 70% of its initial tension after 16 hours when stretched to 30% elongation.
Conventional low-density polyethylene is less than an ideal candidate for stretch pallet wrapping because of its relatively low machine direction ultimate tensile strength and elongation, poor puncture resistance, poor resistance to transverse tear under machine direction tension, and low layer-to-layer tacking. Also, low-density polyethylene generally cannot be produced in thin gauges, particularly from resins having adequate strength and toughness properties.
While ethylene/vinyl acetate copolymers are potential candidates for stretch pallet wrap film materials, those having less than about 4% vinyl acetate content are similar to and exhibit most of the deficiencies of low-density polyethylene. On the other hand, those ethylene/vinyl acetate copolymers which have a vinyl acetate content of more than about 4% are normally of low molecular weight and lack desired physical properties, including ultimate tensile strength, tear resistance, impact resistance, and stress relaxation properties.
Illustrative prior patents relating to stretch wrap films include U.S. Pat. Nos. 3,433,573 and 3,932,563.
In my copending and coassigned U.S. application Ser. No. 961,293, now U.S. Pat. No. 4,222,913, I describe a stretch wrap film which contains 79 to 98% by weight of a high molecular weight ethylene/vinyl acetate copolymer, 1 to 18% by weight of a propylene copolymer, and 0.3 to 5% by weight of a tackifier agent. This blend provides stretch wrap film properties that compare favorably to the prior art with regard to strength, stress relaxation, tear resistance, puncture resistance, and other parameters important for stretch film applications. A need has remained, however, for film possessing even more favorable processability and mechanical properties. In particular, there has been a need for a film having greater elongation with minimal neck-in, a decreased melt strength to permit drawing of thin film, and an increased melting point. There is also a need for such a film having desirable cling properties and, more particularly, such a film which exhibits differential cling, i.e., having one side of the film more tacky than the other. Differential cling characteristics not only facilitate storing of the film on a roll without blocking thereof, but also allow packaging to be carried out with secure adherence of overlapping wraps of the stretch film yet minimize package-to-package adherence that may otherwise cause inconvenience in material handling and even tearing of the packaging film when adjacent packages become stuck to one another and must be separated.
German Offenlegungsschrift No. 28 21 733 describes a film composed of low density polyethylene and polyisobutene which is treated by corona discharge on one surface to develop differential cling, the treated surface thereby becoming more tackey than the untreated surface. However, this material exhibits very limited elongation and other properties desired in a stretch film.